During the oil refining, the LPG before being refined contains a certain amount of sulfides, including inorganic sulfides such as hydrogen sulfide, element sulfur, etc., and organic sulfides such as mercaptans, disulfides, thiophene, etc., wherein the primary active sulfides such as hydrogen sulfide and mercaptans, and nascent-state element sulfur have strong corrosivity, odor and toxicity. In addition, during the process of using the product, these sulfides may be converted into strongly corrosive, toxic and harmful substances, such as sulfur dioxide, sulfur trioxide and the like, which seriously pollute environment and impair human health. Even if the product is used as an intermediate product, the sulfides in trace quantities may result in catalyst poisoning, apparatus corrosion and many other problems. Moreover, the presence of these impurities may result in that LPG cannot pass the copper sheet corrosion test, so that the quality of the product cannot reach the grade as set forth in the national standard and the normal production and the improvement of economic benefit are hereby adversely affected.
At present, the home and oversea processes for refining LPG comprise two procedures: desulfurization and deodorization. The desulfurization procedure generally comprises removing hydrogen sulfide in the LPG obtained by catalytic cracking, delayed coking and hydrogen cracking with alcohol amine solvents, such as monoethanolamine, diethanolamine, N-methyldiethanolamine or diisopropanolamine (hereinafter referred to as alcohol amine treatment). Under the appropriate operation conditions, the level of the residual hydrogen sulfide could be reduced to a trace amount. After alcohol amine treatment, the LPG still generally contains a trace amount of hydrogen sulfide, and, if not removed, it may inactivate the catalyst used in the deodorization procedure and adversely affect the refining effect and the product quality. Thus, the deodorization procedure comprises two steps: one step is to further remove hydrogen sulfide from the LPG treated with the above alcohol amine by prewashing with alkali (generally, sodium hydroxide solution) (also referred to as fine-desulfurization, where hydrogen sulfide reacts with sodium hydroxide to form sodium sulfide), and the alkali solution after the prewashing is recycled and replaced when sodium sulfide therein reaches a certain level; and the other step is to carry out mercaptan conversion of the LPG prewashed with the alkali solution. The method of mercaptan conversion of LPG was firstly proposed by UOP in 1958, and has evolved into the current well-developed liquid-liquid extraction-catalytic oxidization technology. The basic process of said technology comprises dissolving polymeric phthalocyanine cobalt or sulphonated phthalocyanine cobalt as catalyst in sodium hydroxide solution, sufficiently mixing and reacting with LPG in a tower or container, wherein the mercaptans in the LPG react with sodium hydroxide to generate sodium mercaptides that consequently enter the catalyst alkali solution. The reaction equation is:RSH+NaOH→RSNa+H2O
The catalyst alkali solution carrying sodium mercaptides is mixed with air and then enters an oxidization tower to generate disulfides according to the following reaction equation:4RSNa+O2+2H2O→2RSSR+4 NaOH
The disulfides are separated from the catalyst alkali solution by sedimentation in a separating tank, wherein the alkali solution is recycled. However, in practice, the disulfides still substantively remain in the catalyst alkali solution during the above separation, because its commercial-scale process is a continuous process.
It can be seen that the alkali solution used for the prewashing must be frequently replaced during the deodorization, even several times per day. The disulfides formed by oxidizing sodium mercaptides essentially remain in the catalyst alkali solution, so that the catalyst alkali solution must be replaced for ensuring the sulfur content of LPG lower than the standard, after it treated a certain quantity of LPG. Thus, a great quantity of waste alkali solution is generated during the deodorization procedure, and to treat said waste alkali solution (commonly called as alkali sludge) constitutes a heavy burden for oil refining enterprises. The treatment of alkali sludge before discharge generally is a complex and costly process, and often makes secondary pollution. Secondly, the tail gas generated during the mixing and subsequent reaction of sodium-mercaptide containing catalyst alkali solution with air in the oxidization tower not only results in serious environment pollution (as one of the major sources of odors in plant area), but also leads to the lose of materials. Thirdly, said technology uses relatively complex apparatus and process and has higher operation cost.
CN1194294A discloses an alkali-free deodorization process for production of aviation kerosene, comprising mixing raw aviation kerosene with activating agent solution, and passing through a catalyst bed together with air for desulfurization, wherein the catalyst is sulphonated phthalocyanine cobalt. DE19525190A1 discloses a method for removing mercaptans from hydrocarbon distillates such as crude oil distillates, gasoline, kerosene and diesel oil by oxidization, which process comprises converting mercaptans into disulfides with an oxidizing agent in the presence of metal catalyst on carbon fiber fabrics, wherein the used heterogeneous catalyst comprise water-soluble inorganic salts of Cu, Fe, Ni and/or Co, and oxides of Ca, Si, Cu, Mg, Mn, Fe, Zn and/or Al. JP Sho 47-30162B discloses a method of converting mercaptans into disulfides via oxidization. U.S. Pat. No. 5,659,106 discloses a method for removing mercaptans and olefins from petroleum stock by catalytic distillation. CN 1196971 discloses a desulfurizer and a process for preparing the same, wherein the desulfurizer uses Ca2Fe2O5 as its effective component, which can be used for removing hydrogen sulfide from the chemical raw gas derived from coal or petroleum. However, all these documents neither mention the fine desulfurization of LPG by a fixed bed reaction method, nor mention the mercaptan conversion of LPG by a fixed bed catalytic oxidization method.